Sensor for determining radiated energy and use thereof

ABSTRACT

The invention relates to a sensor for determining the energy of radiation of a type that is capable of converting oxygen into ozone, and to a use of such a sensor. According to the invention, the sensor contains a measuring chamber ( 1 ) that can be transirradiated by the radiation and has a gas inlet ( 4 ) and a gas outlet ( 6 ), means ( 8 ) for feeding an oxygen containing gas ( 9 ) into the measuring chamber, via the gas inlet, and for discharging the gas via the gas outlet, one ozone sensor element ( 10 ) for measuring the ozone content of the gas ( 9   a ) located in the measuring chamber or discharged via the gas outlet, and evaluating means ( 12 ) for determining the radiant energy from the measured ozone content. The sensor can be used, for example, to determine radiant energy in an optical imaging system operating with the radiation. Use, for example, in microlithography projection exposure systems.

The invention relates to a sensor for determining the energy ofradiation of a type that is capable of converting oxygen into ozone, andto a use of such a sensor.

Radiant energy sensors are in use in various designs and for variouspurposes, for example, in devices for controlling or regulatingradiation sources of optical imaging systems, in order to set theradiant energy output by the radiation source to a desired, for example,constant value. Such a field of application is photolithographicprojection exposure machines for imaging mask patterns on toresist-coated wafer surfaces in semiconductor technology that operateswith UV radiation. UV radiation belongs to the type of radiation thatconverts oxygen into ozone when the radiation strikes an oxygencontaining gas.

It is known to use photoelectric sensors operating with photodiodes forthe abovementioned purpose of application in photolithographicprojection exposure machines, in order to determine the energy of theradiation used for imaging and, on the basis thereof, to be able to setthe radiant energy to a, for example, constant value, see patents U.S.Pat. Nos. 5.250.797, 5.728.495 and 6.141.081. These photoelectricsensors are used to determine the energy of UV radiation withwavelengths of, for example, 193 nm and 248 nm. The active surface ofsuch sensors is, however, restricted to typically 2 mm×2 mm and istherefore relatively small. A further known type of sensors fordetermining the energy of electromagnetic radiation, specifically alsoin the UV region, are so-called pyrosensors. These are thermal sensorswith a radiation-absorbing layer that heats up upon being irradiated andexpands in the process. The expansion acts on a piezoelectric crystalwhich outputs an electric signal proportional to the thermal expansion.

Both in the case of the photoelectric sensors operating withphotodiodes, and in the case of the pyrosensors, it is usual for thepurpose of measurement for a fraction of the radiation produced by anassociated radiation source to be coupled out as measuring radiation,for example, by means of a beam splitter, and for it to be fed to thesensor. This coupled out fraction of radiation is then no longeravailable for the actual useful radiation function.

The invention is based on the technical problem of providing a sensor ofthe type mentioned at the beginning and a use of the same, which permitsa reliable determination of radiant energy in conjunction with arelatively slight loss of radiation, specifically also for UV radiationwith low wavelengths of 157 nm, for example.

The invention solves this problem by providing a sensor having thefeatures of claim 1, and a use of such as claimed in claim 6.

The sensor according to the invention contains a measuring chamber thatcan be transirradiated by the radiation and has a gas inlet and a gasoutlet, means being provided for feeding an oxygen containing gas intothe measuring chamber via the gas inlet, and for discharging the gas viathe gas outlet. Moreover, the sensor contains one or more ozone sensorelements for measuring the ozone content of the gas located in themeasuring chamber or discharged via the gas outlet. The radiant energyis determined by assigned evaluating means with the aid of the measuredozone content.

The sensor thus designed is suitable for determining the energy ofradiation that, given the presence of oxygen, partially converts atleast some of the latter into ozone. This ozone conversion is dependentin a defined way, for example, an empirically determinable way, on theradiant energy, for example, being proportional thereto. Consequently,when the measuring chamber is fed oxygen containing gas, the fed oxygenis converted at least partially into ozone by such radiation, coupledinto the measuring chamber, the ozone content of the gas still locatedin the measuring chamber or discharged via the gas outlet beingdependent on the radiant energy. Consequently, the evaluating meansdetermine the targeted radiant energy from the measurement of the ozonecontent.

A substantial advantage of this sensor consists in that not all theradiation coupled into the measuring chamber is lost to the actualuseful radiation function, but only that fraction which has contributedto the ozone conversion. The remaining fraction of measuring radiationcan fulfill the envisaged useful function after coupling out from themeasuring chamber.

In accordance with claim 2, in a development of the invention that isadvantageous in terms of design, the measuring chamber is formed by arectilinear measuring tube that can be traversed by the radiation in alongitudinal direction. The radiation can therefore traverse themeasuring chamber rectilinearly without the need for radiationdeflecting means. In addition, for a given measuring chamber volume, arelatively large transirradiation length results, and thus a higherdegree of ozone formation, and this contributes to a high measuringsensitivity for a given quantity of oxygen in the measuring chamber.

In accordance with claim 3, in a further refinement of the inventionthat is advantageous in terms of design the gas inlet and the gas outletare arranged at opposite end regions of the measuring chamber. Thisresults in a correspondingly longer gas flow path through the measuringchamber, and this in turn contributes to an intensive interactionbetween the radiation coupled in and the oxygen contained in the fedgas, and thus to a high rate of ozone formation, and thus to themeasuring sensitivity.

In a development of the invention as claimed in claim 4, the ozonesensor element is advantageously located in the region of a gas outletor of a gas outlet line leading away therefrom such that said ozonesensor element does not disturb the radiation passing through themeasuring chamber, and detects the ozone content of the gas in theregion of the measuring chamber on the gas exit side, which gas containsall the ozone formed by the radiation.

In a development of the invention as claimed in claim 5, the gas feedingmeans are set up for variable setting of the feed rate and/or of theoxygen concentration of the oxygen containing gas. This can be used, forexample, for the purpose of adjusting the measuring sensitivity of thesensor, and thereby of implementing a high measuring range dynamic forthe sensor.

An advantageous use of the sensor according to the invention isprovided, in accordance with claim 6, in an optical imaging systemoperating with the radiation. This can be, in particular, aphotolithographic projection exposure machine. The radiant energy sensorcan serve its purpose here, inside a controller or regulator, ofdetecting the energy, produced by an appropriate radiation source, ofthe radiation used, in particular UV radiation, in order, by means of acontroller or regulator, to set the radiant energy to a specific value,for example to be able to keep it constant.

An advantageous embodiment of the invention is illustrated in thedrawing and is described below.

The sole figure shows a schematic longitudinal sectional view of asensor for determining radiant energy, for example, UV radiation.

The radiant energy sensor shown includes a measuring chamber formed by arectilinear measuring tube 1. The measuring tube is sealed at both endfaces by one radiation transparent window 2, 3 each, which does notabsorb the radiation and is made from CaF₂, for example. Introduced at aslight distance from one end face into the lateral surface of themeasuring tube 1 is a gas inlet 4 into which a gas inlet line 5 opens.Introduced into the lateral surface of the measuring tube 1 in acorresponding way at a slight distance from the other, opposite end faceis a gas outlet 6 from which a gas outlet line 7 leads away. The gasinlet side 4, 5 of the measuring tube 1 is assigned conventional gasfeed means 8, which are shown only schematically in the form of a blockdiagram and can be used to feed pure oxygen or another oxygen containinggas 9 into the gas feed line 5 at a feed rate and/or oxygenconcentration that can be variably set. Positioned in the interior ofthe gas outlet line 7 is a conventional ozone sensor element 10 to whoseelectric measuring signal output an amplifier 11 is connected whoseoutput signal is fed to an evaluating path 12 with an A/D converter andevaluating computer unit. Ozone sensor elements are in use, for example,in the form of so-called semiconductor sensors.

The sensor shown permits the determination of the energy of radiation ofa type that is capable of converting oxygen into ozone by virtue of thefact that the radiation to be measured is guided through the measuringtube 1, in which oxygen-containing gas that has been fed is located, andthe content, dependent on the radiant energy, of ozone formed ismeasured by the ozone sensor element 10.

During use, the radiation 12 to be measured, for example UV radiationwith a wavelength of 157 nm, is coupled into the measuring tube 1 viaone of its end faces by passing through the sealing window 2 there, andsubsequently, traverses the rectilinear measuring tube 1 along itslongitudinal direction and emerges from the measuring tube 1 again atthe opposite end face by passing through the sealing window 3 there. Atthe same time, the gas feed means 8 feed the oxygen containing gas 9 ata desired, controllable feed rate and/or oxygen concentration to themeasuring tube 1 via the gas inlet 4. The oxygen containing gas fedflows in the measuring tube 1 along the longitudinal direction thereofuntil it leaves said tube again via the gas outlet 6. Consequently,while the gas is traversing the measuring tube 1, the radiation 12coupled in is in contact with the oxygen containing gas that is flowingthrough, as a result of which a fraction of the oxygen contained in thegas that is dependent on the radiant energy is converted into ozone.Consequently, the gas 9 a discharged from the measuring tube 1 via thegas outlet 6 has an ozone content that is increased, as a function ofthe radiant energy, by comparison with the fed oxygen containing gasflow 9.

The ozone sensor element 10 detects this ozone content, that is to saythe quantity of ozone formed per unit of time, and passes thisinformation on to the amplifier 11 as an electric signal. The signalamplified by said amplifier is digitized in the evaluating part 12 ofthe A/D converter and then processed by the evaluating computer. Theevaluating computer thereby determines the targeted radiant energy as afunction of the measured ozone content with the aid of the functionaldependence, known to it and for example empirically determinable, ofsaid ozone content, of the energy of the radiation 12 traversing themeasuring tube 1.

It is clear that the radiant energy sensor according to the inventionand described above with the aid of a representative example, issuitable for the most varied fields of application in which the energyof an ozone forming radiation is to be detected, and has severalspecific advantages. An important field of application is the use ofthis radiant energy sensor in optical imaging systems for the purpose ofdetecting, and thereby monitoring the energy of the imaging radiation,and be able to set it to a respectively desired value. Specifically, theradiant energy sensor can be used in photolithographic projectionexposure machines operating with UV radiation, in particular in theirilluminating system. Systems with UV radiation of short wavelengths of,for example, 157 nm have recently been used in this case. There areotherwise few practicable radiant energy sensors for this radiation. Theradiant energy sensor according to the invention permits adequatelyaccurate generation of radiant energy precisely for such UV radiation ofshort wavelengths, since this radiation is absorbed by oxygen to theaccompaniment of strong ozone formation, the ozone formation rate beingproportional to the radiant energy.

An advantage of the radiant energy sensor according to the invention isits high dynamics with a logarithmic signal-to-noise ratio. The point isthat its sensitivity can be regulated over a very wide measuring rangeby appropriate variation of the oxygen flow in the measuring tube 1. Theoxygen flow can be set variably by the gas feed means 8, specifically byvarying the gas feed rate and/or the oxygen concentration in the fedoxygen containing gas 9. Specifically, the radiant energy sensoraccording to the invention can be set to high sensitivity values bycomparison with photoelectric sensors or pyrosensors of conventionaltype. The sensitivity of the ozone sensor element 10 is usuallyessentially constant in this case.

A further great advantage of the radiant energy sensor according to theinvention consists in that it permits the determination of radiantenergy in the traversing beam, that is to say the radiation 12 aemerging from the sensor is available to the system for the purpose offulfilling the actual useful function, in which case it is attenuatedonly slightly in its intensity by comparison with the radiation 12coupled into the sensor, this attenuation being by that fraction whichwas absorbed by the oxygen in the sensor to the accompaniment of ozoneformation. Depending on the case of application, the measuring chamber 1can here be introduced directly into the beam path of the radiation tobe measured, or a fraction of the radiation can be coupled out from themain beam path and then be led through the measuring chamber 1, andsubsequently be coupled into the main beam path again.

The radiant energy sensor according to the invention is not subject toany aging in continuous operation, since freshly fed oxygen containinggas is always flowing through the sensor measuring chamber. The reactiontime of the sensor is determined primarily by that of the ozone sensorelement. For specific applications, as an alternative to this continuousflushing of the measuring chamber by gas, consideration is also given tofeeding the oxygen containing gas only from time to time in a pulsatingfashion during a respective measuring operation, or else not toundertake to feed any fresh oxygen containing gas during the measurementbut to fill the measuring chamber initially with oxygen containing gas,and then to lead the radiation through the measuring chamber andthereafter to measure the ozone content of the gas in the measuringchamber or to flush the measuring chamber and to measure the ozonecontent of the gas driven out of the measuring chamber.

It goes without saying that various modifications of the radiant energysensor shown are possible within the scope of the invention. Thus,depending on application, instead of the rectilinear measuring tube usemay be made of any desired differently shaped measuring chamber that istraversed at least partially by the radiation whose energy is to bedetermined. Moreover, the position of the gas inlet and the gas outletcan be modified at will, and the coupling in and coupling out of theradiation, which is performed via the measuring tube end faces in theexample shown, can likewise be provided at other locations of themeasuring chamber. In this case, it is advantageous in general to have alonger gas transirradiation path referred to the measuring chambervolume, along which the radiation is in contact with the oxygencontaining gas. Instead of being arranged in the gas outlet line, theozone sensor element can also be arranged in the measuring chamberitself preferably in its region on the gas exit side. Moreover, severalozone sensor elements can be positioned at suitable locations ifrequired.

1. A sensor for determining the energy of radiation of a type that iscapable of converting oxygen into ozone, characterized by a measuringchamber (1) that can be transirradiated by the radiation (12) and has agas inlet (4) and a gas outlet (6), means (8) for feeding an oxygencontaining gas (9) into the measuring chamber, via the gas inlet, andfor discharging the gas via the gas outlet, at least one ozone sensorelement (10) for measuring the ozone content of the gas (9 a) located inthe measuring chamber or discharged via the gas outlet, and evaluatingmeans (12) for determining the radiant energy from the measured ozonecontent.
 2. The sensor as claimed in claim 1, further characterized inthat the measuring chamber is formed by a rectilinear measuring tube (1)that can be traversed by the radiation (12) in a longitudinal direction.3. The sensor as claimed in claim 1 or 2, further characterized in thatthe gas inlet (4) and the gas outlet (6) are located at opposite endregions of the measuring chamber (1).
 4. The sensor as claimed in one ofclaims 1 to 3, further characterized in that the ozone sensor element(10) is arranged in the region of the gas outlet (6) or of a gas outletline (7) leading away from said outlet.
 5. The sensor as claimed in oneof claims 1 to 4, further characterized in that the gas feeding means(8) are set up for variable setting of the feed rate and/or of theoxygen concentration of the oxygen containing gas.
 6. A use of thesensor as claimed in one of claims 1 to 5, for determining the radiantenergy in an optical imaging system operating with the radiation (12).